Responses of crops to high temperatures are usually determined under controlled conditions that seldom duplicate field conditions. Our objectives were to devise a system and identify plant traits for evaluating responses to temperature in the field. Seven durum wheat (T. turgidum L. var. durum) cultivars were sown in Morocco during mid and late autumn and early winter to alter timing of maturation. Conditions other than temperature were held as constant as possible by fertilizing and irrigating plants and controlling pests. Plant traits -- photosynthesis, stomatal conductance, yield components, and harvest index -- and their ratios of late versus early plantings were measured during maturation. Mean daily high temperature from anthesis to maturity was 25, 28, and 31 degrees C for the first, second, and third plantings, respectively. The ratio of harvest indices from the third planting relative to the first planting ranged from 0.42 to 1.31, indicating considerable genetic variation among cultivars.